The present invention relates to a device and a method for transmission of information by means of using a spread spectrum technique.
In broadband data transmission, the transmission often has to meet certain requirements, and such requirements can be:
high data rate
short delay
exploitation of a large channel band width
low risk of detection or tapping
good noise immunity
Frequency hopping is a recognized method for creating a spread spectrum having a linear spectrum in an efficient manner, seen over a longer period. Frequency hopping is carried out by means of changing the transmitter and receiver carrier frequency in a predetermined manner. This puts high demands on time synchronization.
Frequency hopping does, however, not guarantee a high momentary band width and thereby high information band width. In order to obtain a high information band width, a modulation method, having a band width adapted to the requirements on information band width, in combination with the frequency hopping, is required.
A very large spread spectrum through frequency hopping also requires long hop sequences, which will result in practical limitations. The noise suppression of the method is directly dependent on the relationship:
signal energy * spread spectrum factor/noise energy
Furthermore, one drawback of the method is that it puts high demands on modulator and de-modulator, respectively, in order to change the frequency fast.
Another problem with this technique is that it is difficult to avoid detection, due to the high power density within a small frequency band, which results in that narrow band receivers also can be used for detecting such on-going traffic.
Another way of obtaining a spread spectrum is by means of direct sequence modulation. Direct sequence modulation is primarily used for obtaining a spread spectrum having a large momentary band width, thereby allowing a high information band width.
The direct sequence modulation is carried out by means of modulating the signal with a long, repeatable, random-like code sequence having a very high autocorrelation function. Since demodulation is carried out in a corresponding manner the signal will be re-created and possible noise will at the same time be suppressed by a factor corresponding to the length of the code sequence, i.e. more efficiently the longer the code sequence or direct sequence length are.
Hence, extreme broadband modulation will require extremely long code sequences, which will result in that, in particular, the demodulator becomes very complex. It can therefore be suitable to implement the demodulator in hardware instead of, which is commonly done, in software. However, also a hardware solution becomes very complex for long code sequences, having large circuit solutions as a result and thereby high costs.
The noise suppression is in the case of direct sequence modulation directly dependent on the relationship:
signal energy * direct sequence length/noise energy
A drawback of this method is that it puts high requirements on the accuracy of the synchronization in the receiver.
A further drawback is that the spectrum of the direct sequence spread spectrum signal is not linear, which reduces the theoretical process gain of the spread spectrum.
By means of combining direct sequence spread spectrum with frequency hop spread spectrum it is possible to obtain a larger spread spectrum than with the methods per se, since the implementations of the methods are limited by different factors, i.e. the limitations described above for the two methods.
In a combination of spread spectrum methods the spread spectrum is formed by the product of the two spread spectrum factors of the applied methods. Typically, the direct sequence modulation can provide for the power of the signal being spread over 10-20 MHz and the frequency can hop in the magnitude of GHz.
However, this technique also has some drawbacks. These mainly consist of higher implementation costs, but also in that even if the spectrum of the signal becomes relatively spread, it will still contain some spikes. This results in that the risk for detection becomes lower than, e.g. for pure frequency hop techniques, but still not minimum, due to the existence of the spikes in the spectrum.
Furthermore, U.S. Pat. No. 5,263,046 describes a spread spectrum technique which can be used for transmission of information by means of simultaneous sweeping from an intermediate frequency to the upper boundary of the channel and from the lower boundary of the channel to said inter-mediate frequency. Information is transmitted by modulating the sweep signals by means of phase switching.
U.S. Pat. No. 5,105,294 describes an optical transmission system which transmits and receives digital ones and zeros, as wave length shifted signals.
Also, U.S. Pat. No. 4,468,792 discloses a method and apparatus for data transmission, using chirped frequency shift keying (FSK) modulation. In order to overcome the problem resulting from i.a. continuous wave (CW) carriers in power line communication systems, the offset frequency of the carrier frequency, representing the information, i.e. being responsive to a particular logic value of a data bit to be transmitted, is swept during the transmission time of the data bit. Thus, by slowly varying the offset frequency in the FSK modulation during transmission of the data bit the interference resulting from CW carriers is reduced.
It is an object of the present invention to provide a method and a device and a transmission system which overcome the problems with the prior art and which at the same time fulfil the requirements mentioned in the introduction, viz. a transmission system which can provide
high data rate
short delay
exploitation of a large channel band width
low risk of detection or tapping
good noise immunity
This object is obtained by transmitting data coded as pre-determined frequency sweeps in relation to a pre-determined frequency, a certain sweep corresponding to a certain symbol. Decoding is then carried out in a device comprising a corresponding number of receiver channels where sweeping reference oscillators, which generate reference signals, are used for verifying the presence of the transmitted, into frequency sweeps coded, symbols. The transmitter then transmits predetermined frequency sweeps during time intervals having a pre-determined duration. The receiver is then able to determine which symbol has been transmitted by means of determining the frequency sweep direction and/or the duration of the frequency sweep.
In order to shorten the time for synchronization in the receiver, each receiver channel can be equipped with a number of reference oscillators, which moreover can be made to follow different frequency sweep signals being displaced in time in relation to each other.
In order to further increase the channel band width and to make tapping and detection more difficult the selected given frequency can also be made to vary according to a pseudo-random scheme. In a preferred embodiment several reference oscillators are provided in each receiver channel, whereby the synchronization time can be reduced and also several, different, in relation to each other delayed, frequency sweep signals can be transmitted in the same frequency band.